GB1567021A

GB1567021A - Scanning transmission microscopes

Info

Publication number: GB1567021A
Application number: GB38325/76A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1975-09-19
Filing date: 1976-09-16
Publication date: 1980-05-08
Also published as: US4097740A; NL7610327A; DE2542356B1; DE2542356C2; JPS5238878A

Description

PATENT SPECIFICATION ( 11) 1 567 021

( 21) Application No 38325/76 ( 22) Filed 16 Sep 1976 ( 19) > ( 31) Convention Application No 2542356 ( 32) Filed 19 Sep 1975 in, ( 33) Fed Rep of Germany (DE)

> ( 44) Complete Specification Published 8 May 1980

L ( 51) INT CL 3 GO 5 D 3/00 HO 1 J 37/21 ( 52) Index at Acceptance G 3 N 277 B 277 X 371 381 385 E 1 X ( 54) IMPROVEMENTS IN OR RELATING TO SCANNING TRANSMISSION MICROSCOPES ( 71) We, SIEMENS AKTIENGESELLSCHAFT, a German Company of Berlin and Munich, German Federal Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement:-

The invention relates to scanning transmission microscopes in which the beam is guided 5 by a deflection system excited in sawtooth form such that it describes a parallel line raster on the object to be examined, and in which a detector is provided in the beam path behind the object, the output of said detector controlling the brightness of an image tube monitor operated in synchronisation with the raster A scanning transmission electron microscope of this type is described, for instance, in the periodical "Journal of Applied Physics", Vol 39, 10 no 13, 1968, page 5861 onwards.

One object of the present invention is to provide means for bringing the greatly diminished image of the beam source projected by the microscope objective lens into the plane in which the object being examined lies with maximum possible precision through i 5 corresponding adjustment of the lens current 15 The invention consists in a scanning transmission microscope in which means are provided for making a focussing adjustment of the objective lens, the beam being guided by a deflection system excited in sawtooth form such that it describes a parallel line raster on an object to be examined, and detector means being provided in the beam path behind the object, the output of said detector means controlling the brightness of an image tube 20 monitor operated in synchronisation with the raster, and said detector means also being capable of measuring the intensity of beam components while a point of the object is being scanned in a cone of the beam that has passed through the object at two points positioned symmetrically with respect to the cone axis, said detector means having an effective entry area smaller than the cross-section of the cone at the same plane, and means being provided 25 to set the lens current of the objective lens so that the output magnitudes of the detector means are the same for both measurements.

The invention makes use of the knowledge that with too short a focal length relative to the object (over-focussing) or with too long a focal length (underfocussing) the beam cone that has passed through the object projects a silhouette of a more or less extended object 30 zone in the plane of the detector This silhouette is structured; in other words the intensity components that can be measured in the beam cone, symmetrically on each side of the axis are usually different unless the focal spot is one the object, in which case the beam intensity in the cone is unstructured, so that the intensities measured at symmetrical points on the cone cross-section are the same However this applies strictly to noncrystalline objects, and 35 consequently it may be advantageous when studying crystalline objects to use a test object such as the carrier film which is generally of amorphous carbon in a preliminary focussing adjustment process.

Two detectors may be provided, lying symmetrical with respect to the optical axis for the measuring of the beam components, each of these having a small entry area relative to the 40 cone cross-section After amplification the output magnitudes of the detectors can be read directly from instruments, the objective lens current being adjusted until the difference indicated on the two instruments reaches a minimum.

The cost in terms of apparatus is generally less if a single detector lying on the optical axis is used for the measuring, and a secondary deflection system is provided by which the cone 45 1 567 021 L of the beam after the object is deflected alternately in two opposed equal angles relative to the optical axis, a deflection system excited with a rectangular waveform A C supply.

In both cases the measurement can be carried out while the deflection system serving to generate the object raster is not excited, i e while an object point is being irradiated by a beam in its rest position In the second case the measurement can also be utilised during the 5 production of an object raster if the frequency of the secondary deflection of the beam cone under the object is high relative to the line frequency of the object raster This means that the time interval between two deflections of the beam cone is roughly the same as the spot time of the object raster, so that here again the detector output magnitudes are compared while one object point is being scanned 10 With the latter embodiment, the measurement by the single axial detector output magnitude can be advantageously reduced to an alternating current measurement by amplifying the detector output with a narrow-band amplifier tuned to the frequency of the deflection system serving to deflect the beam cone under the object In addition the lens current can be overlaid with an alternating current of low frequency relative to the 15 deflection frequency, the phase difference of this alternating current and the modulation of the detector output magnitude being used as a criterion for the direction of the required lens current modification.

The invention will now be described with reference to the drawings, in which:Figure 1 schematically illustrates one exemplary embodiment of a scanning transmission 20 microscope, together with details of an alternative embodiment, and Figures 2 and 3 are explanatory waveform diagrams.

In the embodiment schematically illustrated in Figure 1, a beam source 1 of the microscope, which may comprise a field-emission cathode, directs a beam on axial path A through a deflection system 3 with sequential stages 3 a and 3 b that serve to produce a 25 scanning raster in the plane of an object to be studied Each stage 3 a, or 3 b, consists of two mutually perpendicularly disposed pairs of electrostatic deflection plates or magnetic deflection coils to deflect the beam 2 in two vertically opposed directions x and y, and in the embodiment shown, which uses coils, only the relevant pairs for direction x are shown The deflection system 3 is excited by a raster generator RG The stage 3 a deflects the beam 2 30 away from the axis A while stage 3 b deflects the beam back to the axis A again, along a path 2 a.

The beam is focussed on the object 5 by an electromagnetic objective lens 4 and pivoted by the deflection system 3 about a point P which lies in the focal plane of the objective lens 4 A further deflection system 6 and a detector D are preferably disposed under the object 35 5, as will be described hereinafter, the deflection system consisting of just one pair of deflection plates or coils.

A detector D has its output connected via an amplifier V 1 to a brightness control of an image tube monitor 10, the deflection system of which is also controlled by the raster generator RG 40 Figure 1 illustrates a microscope state in which, as a result of excessive excitation of the objective lens 4, the focal spot F of the beam 2 does not fall on an object 5, as it should, but in front of the object in the beam path Consequently the beam produces a silhouette projection of an object zone 5 with one downward-directed cone 2 b; in other words a silhouette of the zone 5 a is produced in an entry aperture d lying in a plane 8 at the entry 45 face of the detector D The entry aperture d is smaller than the crosssection of the beam cone in the plane 8; in other words the detector apertural angle a D is smaller than the illumination apertural angle a B. It is clear that the silhouette of the object zone 5 a produced in the plane 8 is structured just like the object zone itself The same is true in the event that the focal spot F lies below 50 the object 5 with the objective lens 4 insufficiently excited; here again a structured image appears in the plane 8 But if the focal spot F lies on the object 5, the beam cone under the object is not structured.

Firstly, it can be assumed that the deflection system 3 is not excited, so that the beam 2 follows an axial path and always illuminates the same point or zone of the object 5 If the 55 deflection system 6 is now alternatively excited in two discrete and different states, so that the cone 2 b is successively deflected to opposite sides of the axis A and the axis of the cone forms opposing equal angles with the axis A, then in the two deflection states parts of the cone lying symmetrical with respect to the cone axis act upon the relatively small aperture d of the detector D The beam intensity components which the detector D measures in the 60 process are different if the focussing is incorrect because of the irregular structuring of the silhouette; but they are the same when the focal spot lies on the object 5 and the cone 2 b exhibits no structuring.

Therefore it is possible to establish directly by measuring the output magnitudes of detector D in both deflection states whether the focal spot F is lying on the object 5 or not 65 1 567 021 As a result of this measurement the excitation of the objective lens 4 can be adjusted, if necessary, so that the output magnitudes of detector D are the same in both states The focal spot F then lies on the object.

In an alternative embodiment one can also proceed in basically the same way, without using the deflection system 6, providing that two detectors d' are disposed symmetrically 5 with respect to the axis A in the plane 8 In this case as well comparison of the output magnitudes of the two detectors provides a criterion for the position of the focal spot F.

In the following the operation of the arrangement for automatic implementation of focussing 1 is described.

A lens current regulator LR is provided for the power supply to the objective lens 4, and 10 this regulator consists of a regulating part L which supplies a temporally constant control magnitude, and a regulating part L' which emits an additional control magnitude in the positive or negative direction which is also temporally constant Both magnitudes control an amplifier V 2 which supplies the current for the objective lens 4 Provision is also made for a lens sweep generator LWG which makes it possible to superimpose a sinusoidal alternating 15 current on the lens current The regulating part L can be adjusted manually by an operating control 11.

The deflection system 6 is excited by a deflecting sweep generator AWG which supplies a rectangular alternating current This causes the cone 2 b to be deflected alternately into two positions symmetrically with respect to the axis A The frequency of the deflecting sweep 20 generator AWG is high relative to the line frequency of the raster generator RG, with which the x-direction of the deflection system 3 is operated This means that two successive deflection states of cone 2 b are associated with the same point or zone of the object 5, in other words successive deflections effectively take place during one image spot time The frequency of the deflecting sweep generator AWG is also high in relation to the frequency 25 of the lens sweep generator LWG.

The output of the detector amplifier V 1 is fed via a narrow-band amplifier SBV, which has a gating input connected to the output of the deflecting sweep generator AWG, so that the amplifier SBV is triggered by AWG The narrow-band amplifier SBV is therefore tuned 3 to the frequency of AWG The output signal 51 is fed to a phase discriminator PD, a second 30 input of which is connected to the output of the lens sweep generator LWG The phase discriminator PD supplies a positive or a negative signal to the regulating part L', depending on whether LWG and 51 are of the same or of opposing phase The regulating part L' then either increases or reduces the current to the objective lens 4.

31 Figure 2 shows typical patterns of the signal 51 emitted by the narrowband amplifier 35 SBV for different focussing states, and Figure 3 shows the cycle of the lens sweep generator LWG in relation to time.

Leaving any lens sweep out of consideration for the present, in the event of over or under-focussing, the signal 51 consists of a voltage whose magnitude "a" (Figure 2) corresponds to the absolute difference in the detector output magnitudes in both positions 40 of the cone If the lens current is now made to fluctuate at a low frequency (cf Figure 3), the signal 51 is modulated by the lens sweep process This modulation is characterised by different phases relative to the lens sweep according to whether there is over-focussing (Figure 2 a) or under-focussing (Figure 2 c) With energising direct current to the lens 4 correctly adjusted, the maximum distances between the focal spot and the object 5 during 45 the lens sweep are the same so that any modulation of the signal 51 as in Figure 2 b is made up of two half-waves, of which one is in phase with the lens sweep and the other has the opposite phase relative thereto Therefore, with the signals 51 as in Figures 2 a and 2 c, the phase discriminator PD supplies opposing signals, the sign of which corresponds to the so direction of the focal discrepancy to the lens controller, whereas it emits no signal at all in 50 the case represented by Figure 2 b After the correct focus has been set the lens sweep generator LWG and the deflecting sweep generator AWG are switched off during normal operation of the microscope.

It should be expressly pointed out that the illustrated automatic arrangement works effectively during normal scanning excitation of the deflection system 3, i e during the 55 normal production of a raster on the object 5 Consequently, an image of the resolved object zone can be seen on the screen of the image tube monitor 10 As long as the focal spot F does not lie in the plane of the object, this image is a double image as indicated in Figure 1, but becomes a single image when the focussing is correct Therefore it is possible for an operator to check the state of focussing on the monitor screen visually 60 In the following a few operating magnitudes are indicated for the full arrangement shown in Figure 1, by way of examples:1 567 021 Illumination apertural angle % 2 10-2 rad Detector apertural angle a D 5 10-3 rad Raster generator RG: S Image time Timage = 4 sec Line time tline = 20 msec 10 Image spot time TBP 100 lisec Deflecting sweep generator AWG:1 S Oscillation time T U= 50 psec IS Lens sweep generator LWG:Oscillation time To-'= 40 msec.

20 The described arrangement can be employed for astigmatic correction of an objective lens, since this basically entails focussing correction in two different planes containing the optical axis For this purpose, for example, the deflection system 6 can be rotated orbitally by additional mechanical or electrical means (not shown) and a stigmator (not shown) can be set up so that the lens current set for correct focussing is the same for all azimuthal 25 directions In this case a further pair of deflection coils would be required for the electrical rotation of the deflection system 6.

The scanning transmission microscope may employ a beam of electrons or of ions.

Claims

WHAT WE CLAIM IS:-

1 A scanning transmission microscope in which means are provided for focussing 30 adjustment of the objective lens, the beam being guided by a deflection system excited in saw-tooth form such that it describes a parallel line raster on an object to be examined, and detector means being provided in the beam path behind the object, the output of said detector means controlling the brightness of an image tube monitor operated in synchronisation with the raster, and said detector means also being capable of measuring 35 the intensity of beam components while a point on the object is being scanned in a cone of the beam that has passed through the object at two points positioned symmetrically with respect to the cone axis, said detector means having an effective entry area smaller than the cross-section of the cone at the same plane, and means being provided to set the lens current of the objective lens so that the output magnitudes of the detector means are the 40 same for both measurements.

2 A microscope as claimed in Claim 1, in which the detector means comprises two detectors, lying symmetrical about the optical axis.

3 A microscope as claimed in Claim 1, in which said detector means lies on the optical axis and said cone is deflected alternately along paths at respective opposed equal angles 45 relative to the optical axis by a secondary deflection system after the object on the beam path and energised by a rectangular waveform current, the deflection frequency being high relative to the line frequency of the object raster.

4 A microscope as claimed in Claim 3, in which said detector output is amplified by a narrow-band amplifier tuned to the frequency of said secondary deflection system 50 A microscope as claimed in Claim 3 or Claim 4, in which an alternating current of low frequency relative to the deflection frequency is superimposed on the current to the objective lens, and that the phase difference of this alternating current on the one hand and the modulation of the detector output magnitude on the other is used as a criterion for the direction of the required lens current modification for focussing said lens 55 6 A microscope as claimed in Claim 5, in which said detector means lies on the optical axis behind the object and said secondary deflection system, and a deflecting sweep generator energising said secondary deflection system, produces a rectangular output waveform, when operating, to deflect the beam cone in two directions symmetrically with respect to the optical axis, the frequency of which output is high relative to the line 60 frequency of the object raster and in which a lens current regulating arrangement is controlled by a lens current sweep generator acting on the lens current regulating arrangement to superimpose an alternating current on the current to the objective lens whilst a phase discriminator serves for phase comparison between said lens current sweep generator and said detector output magnitude, which then produces a control signal for said 65 A 1 567 021 5 lens current regulating arrangement.

7 A microscope as claimed in any one of Claims 1 to 4, in which said adjustment is astigmatic correction of the objective lens.

8 A corpuscular beam scanning transmission microscope substantially as described with reference to Figure 1 5 For the Applicants, G F REDFERN & CO, Marlborough Lodge, 14 Farncombe Road, Worthing West Sussex.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1980.

Published by The Patent Office, 25 Southampton Buildings London WC 2 A IA Yfrom which copies may be obtained.